1. Field of the Disclosure
The present invention relates generally to transmission devices, and more particularly to transmit power control in transmission devices.
2. Description of the Related Art
Transmission devices often utilize control algorithms to control the transmission power, i.e., the power used in transmitting signals. In general, the term “transmit power control” refers to the intelligent selection of transmit power in a communication system to achieve good (or desired) performance within the system. The notion of “good performance” can depend on the system context and may include optimizing metrics such as signal to noise ratio, link data rate, network capacity, geographic coverage and range, and power consumption, among others. Transmit power control algorithms are used in many areas, including wireless LANs, cellular networks, etc.
A transmit power control algorithm will often increase the transmit power of the transmitter in some situations to provide a higher signal power at the receiver. The higher signal-to-noise ratio (SNR) at the receiver results in better reception at the receiver, i.e., results in a reduced bit error rate on the digital communication link. An increased transmit power can also be used to achieve a higher data rate of transmission across the communication link. For example, a higher SNR may allow a system that uses link adaptation to transmit at a higher data rate, thus achieving greater spectral efficiency. Another potential benefit is that an increased transmit power for a given modulation typically yields an increased signal range. In a cellular network, a higher transmit power results in a lower probability of dropped calls. In a wireless system having a fading channel, a higher transmit power provides greater protection against signal fade.
However, using a higher transmit power has various drawbacks. For example, a higher transmit power increases the overall power consumption of the device. This is of particular concern in mobile devices, where battery life is very important. Also, a higher transmit power increases the interference to other users in the same frequency band, which is undesirable.
Power control algorithms are often designed to accommodate the performance criteria of the designer. Power control is generally a difficult problem, as power control algorithms must strike a balance between the benefits of increased SNR and the drawbacks of increased power consumption and signal interference.
It is often a challenge to design a transmit power control (TPC) algorithm to achieve absolute accuracy. This is because output power can be affected by variations in process, temperature, frequency, voltage, soldering, and board component tolerances, among others.
Closed loop power control schemes attempt to use feedback loops and power detectors to achieve a desired accuracy. However, when used, the power detectors themselves typically have to be calibrated. Further, some manufacturers are including wireless communication chips direct on board (DoB), which in some designs makes calibration with an external power meter undesirable or impossible.
Another method to achieve power control accuracy is to perform open loop power control with calibration to reduce chip (process) variation. However, such calibration is challenging to get correct, and does not correct for board to board variation.
Therefore, improvements in power control algorithms are desired.